tests: switch quadlet test to use :latest fedora tag

This test is just sleeping so failure risk is minimal on a
major container update. Let's just use latest.

Since the container definition is in the butane config it's harder
to dynamically update this so let's just use a moving tag. As
mentioned above the risk is minimal.

tests/kola/containers/quadlet/config.bu

@@ -9,7 +9,7 @@ storage:
           Description=A minimal container
 
           [Container]
-          Image=quay.io/fedora/fedora-minimal:40
+          Image=quay.io/fedora/fedora-minimal:latest
           Exec=sleep 60
           Volume=test.volume:/data
           Network=test.network

tests/kola/containers/quadlet/test.sh

@@ -32,7 +32,7 @@ if ! is_service_active test.service; then
   fatal "test-network.service failed to start"
 fi
 container_info=$(podman container inspect systemd-test)
-if [[ "$(jq -r '.[0].ImageName' <<< "$container_info")" != "quay.io/fedora/fedora-minimal:40" ]]; then
+if [[ "$(jq -r '.[0].ImageName' <<< "$container_info")" != "quay.io/fedora/fedora-minimal:latest" ]]; then
     fatal "Container not using the correct image"
 fi
 if [[ "$(jq -r '.[0].NetworkSettings.Networks[].NetworkID' <<< "$container_info")" != "systemd-test" ]]; then